Determination of the free core nutation period from tidal gravity observations of the GGP superconducting gravimeter network

Abstract

This study is based on 25 long time-series of tidal gravity observations recorded with superconduct- ing gravimeters at 20 stations belonging to the Global Geodynamic Project (GGP). We investigate the diurnal waves around the liquid core resonance, i.e., K1, ψ1 and ϕ1, to determine the free core nutation (FCN) period, and compare these experimental results with models of the Earth response to the tidal forces. For this purpose, it is necessary to compute corrected amplitude factors andphasedifferencesbysubtractingtheoceantideload- ing (OTL) effect. To determine this loading effect for each wave, it was thus necessary to interpolate the con- tribution of the smaller oceanic constituents from the four well determined diurnal waves, i.e., Q1,O1,P1,K1. It was done for 11 different ocean tide models: SCW80, CSR3.0, CSR4.0, FES95.2, FES99, FES02, TPXO2, ORI96, AG95, NAO99 and GOT00. The numerical re- sults show that no model is decisively better than the others and that a mean tidal loading vector gives the most stable solution for a study of the liquid core res- onance. We compared solutions based on the mean of the 11 ocean models to subsets of six models used in a previous study and five more recent ones. The cali- bration errors put a limit on the accuracy of our global results at the level of ±0.1%, although the tidal factors of O1 and K1 are determined with an internal precision of close to 0.05%. The results for O1 more closely fit